Conventionally, in a proportional solenoid control valve using an electromagnetic proportional solenoid, components configured to be movable, such as a spool (hereinafter, such components may be collectively referred to as “movable components”), are driven by the driving force of the solenoid in such a direction as to oppose to spring force or hydraulic force so as to control the positions of the movable components and the balance of the force, and thereby a flow rate or pressure is controlled.
As shown in FIG. 6, such an electromagnetic proportional solenoid as described above is configured to generate driving force proportional to an applied current value (control current value), and the aforementioned flow rate or pressure is controlled in accordance with the applied current. FIG. 6 shows an example where solenoid driving force F3 is generated when the value of the applied current is a rated value. For example, in the case of a proportional solenoid directional flow control valve 101 in a first example shown in FIGS. 7A and 7B, FIG. 7A shows a spool 105 located at a neutral position in a standby state. The spool 105 is a movable component movable in an axial direction inside a spool hole 106 formed in a valve body 102. When the spool 105 is in the standby state, if an electromagnetic proportional solenoid 103 is driven as shown in FIG. 7B, then the spool 105 moves in a driving direction V, so that driving force generated by the electromagnetic proportional solenoid 103 becomes equivalent to the spring force of a spool-returning spring 104. In this manner, the degree of opening between a pump port P and an output port A is controlled, and the flow rate from the pump port P to the output port A is controlled in accordance with the applied current. When the spool 105 is returned to the standby state, the output port A comes into communication with a tank port T. The same is true of, for example, a second example shown in FIGS. 8A and 8B where electromagnetic proportional solenoids 123 are provided at both sides of a proportional solenoid control valve 121, i.e., dual-drive system. In the second example, a spool 125 is driven so as to be located at a position where driving force in the driving direction V, the driving force being generated by one of the electromagnetic proportional solenoids 123, becomes equivalent to the spring force of a corresponding one of spool-returning springs 124. In FIGS. 8A and 8B, the same components as the components shown in FIGS. 7A and 7B are denoted by reference numerals that are greater by 20 than the reference numerals of the components shown in FIGS. 7A and 7B, and the description of such common components is omitted.
As another example, in the case of a pilot-type proportional solenoid flow control valve capable of accommodating a great flow rate, control pressure generated by a proportional solenoid pressure-reducing valve configured to control pressure in accordance with an applied current is introduced as pilot pressure into a spring chamber of a main spool. Then, the main spool is driven so as to be located at a position where force derived from the pilot pressure and opposing spring force of a spool-returning spring become equivalent to each other. In this manner, the main spool is driven in an axial direction, and thereby the degree of opening of an oil passage is controlled, such that the flow rate is controlled in accordance with the applied current.
As shown in FIG. 9, generally speaking, an oscillating wave is used as a current applied to the proportional solenoid of such a proportional solenoid control valve. The value of the current varies relative to time. The oscillating current is called current dither, which is an oscillating wave with a predetermined amplitude. Through the application of the current dither, movable components inside the solenoid and movable components driven by the solenoid are caused to always micro-vibrate in the axial direction, so that the sliding friction of the movable components is reduced and hysteresis is reduced.
Moreover, the current dither causes the spool to always micro-vibrate in the axial direction, thereby suppressing hydraulic lock from occurring and always keeping the operation of movable components in a favorable state. The hydraulic lock is a malfunctioning (failed returning) state of the spool. In the hydraulic lock, the spool becomes eccentric with respect to the spool hole; the spool in such an eccentric state is pressed against the side surface of the spool hole by hydraulic pressure; and due to the pressing in the eccentric direction, the spool becomes unable to move in the axial direction. In this respect, also in the case of the aforementioned pilot-type proportional solenoid control valve, movable components of the proportional solenoid pressure-reducing valve micro-vibrate. As a result, the pilot pressure vibrates, and the vibration of the pilot pressure causes micro vibration of the main spool. Consequently, the aforementioned effect of reducing the sliding friction of movable components and effect of suppressing the hydraulic lock can be obtained.
As described above, proportional solenoid control valves are configured such that current dither is added to the applied current for the purpose of reducing hysteresis in control characteristics and preventing malfunctioning.
One of the conventional techniques of the above kind is such that, so a solenoid control valve configured to control a main spool by means of a proportional solenoid pressure-reducing valve, the main spool is formed of a light metal and the surface of the main spool is hardening-treated so that favorable hysteresis performance can be obtained (see Patent Literature 3, for example).
As another example of conventional techniques, there is an operating valve provided with a proportional solenoid pressure-reducing valve configured to drive a spool included in a valve body (see Patent Literature 2, for example).